TABLE OF CONTENTS

sshd(8) reads configuration data
from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The
file contains keyword-argument pairs, one per line. For each keyword, the
first obtained value will be used. Lines starting with
‘#’ and empty lines are interpreted as
comments. Arguments may optionally be enclosed in double quotes (") in
order to represent arguments containing spaces.

The possible keywords and their meanings are as follows (note that keywords are
case-insensitive and arguments are case-sensitive):

Specifies what environment variables sent by the client will be copied
into the session's
environ(7). See
SendEnv and
SetEnv in
ssh_config(5) for how to
configure the client. The TERM
environment variable is always accepted whenever the client requests a
pseudo-terminal as it is required by the protocol. Variables are specified
by name, which may contain the wildcard characters
‘*’ and
‘?’. Multiple environment variables
may be separated by whitespace or spread across multiple
AcceptEnv directives. Be warned that
some environment variables could be used to bypass restricted user
environments. For this reason, care should be taken in the use of this
directive. The default is not to accept any environment variables.

Specifies whether
ssh-agent(1) forwarding
is permitted. The default is yes. Note
that disabling agent forwarding does not improve security unless users are
also denied shell access, as they can always install their own
forwarders.

This keyword can be followed by a list of group name patterns, separated
by spaces. If specified, login is allowed only for users whose primary
group or supplementary group list matches one of the patterns. Only group
names are valid; a numerical group ID is not recognized. By default, login
is allowed for all groups. The allow/deny directives are processed in the
following order: DenyUsers,
AllowUsers,
DenyGroups, and finally
AllowGroups.

Specifies whether StreamLocal (Unix-domain socket) forwarding is
permitted. The available options are
yes (the default) or
all to allow StreamLocal forwarding,
no to prevent all StreamLocal
forwarding, local to allow local (from
the perspective of ssh(1))
forwarding only or remote to allow
remote forwarding only. Note that disabling StreamLocal forwarding does
not improve security unless users are also denied shell access, as they
can always install their own forwarders.

Specifies whether TCP forwarding is permitted. The available options are
yes (the default) or
all to allow TCP forwarding,
no to prevent all TCP forwarding,
local to allow local (from the
perspective of ssh(1))
forwarding only or remote to allow
remote forwarding only. Note that disabling TCP forwarding does not
improve security unless users are also denied shell access, as they can
always install their own forwarders.

This keyword can be followed by a list of user name patterns, separated by
spaces. If specified, login is allowed only for user names that match one
of the patterns. Only user names are valid; a numerical user ID is not
recognized. By default, login is allowed for all users. If the pattern
takes the form USER@HOST then USER and HOST are separately checked,
restricting logins to particular users from particular hosts. HOST
criteria may additionally contain addresses to match in CIDR
address/masklen format. The allow/deny directives are processed in the
following order: DenyUsers,
AllowUsers,
DenyGroups, and finally
AllowGroups.

Specifies the authentication methods that must be successfully completed
for a user to be granted access. This option must be followed by one or
more lists of comma-separated authentication method names, or by the
single string any to indicate the
default behaviour of accepting any single authentication method. If the
default is overridden, then successful authentication requires completion
of every method in at least one of these lists.

For example, “publickey,password
publickey,keyboard-interactive” would require the user to complete
public key authentication, followed by either password or keyboard
interactive authentication. Only methods that are next in one or more
lists are offered at each stage, so for this example it would not be
possible to attempt password or keyboard-interactive authentication before
public key.

For keyboard interactive authentication it is also possible to restrict
authentication to a specific device by appending a colon followed by the
device identifier bsdauth,
pam, or
skey, depending on the server
configuration. For example, “keyboard-interactive:bsdauth”
would restrict keyboard interactive authentication to the
bsdauth device.

If the publickey method is listed more than once,
sshd(8) verifies that keys
that have been used successfully are not reused for subsequent
authentications. For example, “publickey,publickey” requires
successful authentication using two different public keys.

Note that each authentication method listed should also be explicitly
enabled in the configuration.

The available authentication methods are: “gssapi-with-mic”,
“hostbased”, “keyboard-interactive”,
“none” (used for access to password-less accounts when
PermitEmptyPasswords is enabled),
“password” and “publickey”.

Specifies a program to be used to look up the user's public keys. The
program must be owned by root, not writable by group or others and
specified by an absolute path. Arguments to
AuthorizedKeysCommand accept the tokens
described in the TOKENS
section. If no arguments are specified then the username of the target
user is used.

The program should produce on standard output zero or more lines of
authorized_keys output (see
AUTHORIZED_KEYS in
sshd(8)). If a key supplied by
AuthorizedKeysCommand does not
successfully authenticate and authorize the user then public key
authentication continues using the usual
AuthorizedKeysFile files. By default,
no AuthorizedKeysCommand is run.

Specifies the user under whose account the
AuthorizedKeysCommand is run. It is
recommended to use a dedicated user that has no other role on the host
than running authorized keys commands. If
AuthorizedKeysCommand is specified but
AuthorizedKeysCommandUser is not, then
sshd(8) will refuse to
start.

Specifies the file that contains the public keys used for user
authentication. The format is described in the
AUTHORIZED_KEYS
FILE FORMAT section of
sshd(8). Arguments to
AuthorizedKeysFile accept the tokens
described in the TOKENS
section. After expansion,
AuthorizedKeysFile is taken to be an
absolute path or one relative to the user's home directory. Multiple files
may be listed, separated by whitespace. Alternately this option may be set
to none to skip checking for user keys
in files. The default is “.ssh/authorized_keys
.ssh/authorized_keys2”.

Specifies a program to be used to generate the list of allowed certificate
principals as per
AuthorizedPrincipalsFile. The program
must be owned by root, not writable by group or others and specified by an
absolute path. Arguments to
AuthorizedPrincipalsCommand accept the
tokens described in the TOKENS
section. If no arguments are specified then the username of the target
user is used.

The program should produce on standard output zero or more lines of
AuthorizedPrincipalsFile output. If
either AuthorizedPrincipalsCommand or
AuthorizedPrincipalsFile is specified,
then certificates offered by the client for authentication must contain a
principal that is listed. By default, no
AuthorizedPrincipalsCommand is
run.

Specifies the user under whose account the
AuthorizedPrincipalsCommand is run. It
is recommended to use a dedicated user that has no other role on the host
than running authorized principals commands. If
AuthorizedPrincipalsCommand is
specified but
AuthorizedPrincipalsCommandUser is not,
then sshd(8) will refuse to
start.

Specifies a file that lists principal names that are accepted for
certificate authentication. When using certificates signed by a key listed
in TrustedUserCAKeys, this file lists
names, one of which must appear in the certificate for it to be accepted
for authentication. Names are listed one per line preceded by key options
(as described in
AUTHORIZED_KEYS
FILE FORMAT in sshd(8)).
Empty lines and comments starting with
‘#’ are ignored.

Arguments to AuthorizedPrincipalsFile
accept the tokens described in the
TOKENS section. After
expansion, AuthorizedPrincipalsFile is
taken to be an absolute path or one relative to the user's home directory.
The default is none, i.e. not to use a
principals file – in this case, the username of the user must
appear in a certificate's principals list for it to be accepted.

Note that AuthorizedPrincipalsFile is
only used when authentication proceeds using a CA listed in
TrustedUserCAKeys and is not consulted
for certification authorities trusted via
~/.ssh/authorized_keys, though the
principals= key option offers a similar
facility (see sshd(8) for
details).

Specifies the pathname of a directory to
chroot(2) to after
authentication. At session startup
sshd(8) checks that all
components of the pathname are root-owned directories which are not
writable by any other user or group. After the chroot,
sshd(8) changes the working
directory to the user's home directory. Arguments to
ChrootDirectory accept the tokens
described in the TOKENS
section.

The ChrootDirectory must contain the
necessary files and directories to support the user's session. For an
interactive session this requires at least a shell, typically
sh(1), and basic
/dev nodes such as
null(4),
zero(4),
stdin(4),
stdout(4),
stderr(4), and
tty(4) devices. For file
transfer sessions using SFTP no additional configuration of the
environment is necessary if the in-process sftp-server is used, though
sessions which use logging may require
/dev/log inside the chroot directory on
some operating systems (see
sftp-server(8) for
details).

For safety, it is very important that the directory hierarchy be prevented
from modification by other processes on the system (especially those
outside the jail). Misconfiguration can lead to unsafe environments which
sshd(8) cannot detect.

Specifies the ciphers allowed. Multiple ciphers must be comma-separated.
If the specified value begins with a ‘+’ character, then the
specified ciphers will be appended to the default set instead of replacing
them. If the specified value begins with a ‘-’ character,
then the specified ciphers (including wildcards) will be removed from the
default set instead of replacing them.

Sets the number of client alive messages which may be sent without
sshd(8) receiving any messages
back from the client. If this threshold is reached while client alive
messages are being sent, sshd will disconnect the client, terminating the
session. It is important to note that the use of client alive messages is
very different from TCPKeepAlive. The
client alive messages are sent through the encrypted channel and therefore
will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The client
alive mechanism is valuable when the client or server depend on knowing
when a connection has become inactive.

The default value is 3. If
ClientAliveInterval is set to 15, and
ClientAliveCountMax is left at the
default, unresponsive SSH clients will be disconnected after approximately
45 seconds.

Sets a timeout interval in seconds after which if no data has been
received from the client,
sshd(8) will send a message
through the encrypted channel to request a response from the client. The
default is 0, indicating that these messages will not be sent to the
client.

This keyword can be followed by a list of group name patterns, separated
by spaces. Login is disallowed for users whose primary group or
supplementary group list matches one of the patterns. Only group names are
valid; a numerical group ID is not recognized. By default, login is
allowed for all groups. The allow/deny directives are processed in the
following order: DenyUsers,
AllowUsers,
DenyGroups, and finally
AllowGroups.

This keyword can be followed by a list of user name patterns, separated by
spaces. Login is disallowed for user names that match one of the patterns.
Only user names are valid; a numerical user ID is not recognized. By
default, login is allowed for all users. If the pattern takes the form
USER@HOST then USER and HOST are separately checked, restricting logins to
particular users from particular hosts. HOST criteria may additionally
contain addresses to match in CIDR address/masklen format. The allow/deny
directives are processed in the following order:
DenyUsers,
AllowUsers,
DenyGroups, and finally
AllowGroups.

Writes a temporary file containing a list of authentication methods and
public credentials (e.g. keys) used to authenticate the user. The location
of the file is exposed to the user session through the
SSH_USER_AUTH environment variable. The
default is no.

Forces the execution of the command specified by
ForceCommand, ignoring any command
supplied by the client and ~/.ssh/rc if
present. The command is invoked by using the user's login shell with the
-c option. This applies to shell, command, or subsystem execution. It is
most useful inside a Match block. The
command originally supplied by the client is available in the
SSH_ORIGINAL_COMMAND environment
variable. Specifying a command of
internal-sftp will force the use of an
in-process SFTP server that requires no support files when used with
ChrootDirectory. The default is
none.

Specifies whether remote hosts are allowed to connect to ports forwarded
for the client. By default,
sshd(8) binds remote port
forwardings to the loopback address. This prevents other remote hosts from
connecting to forwarded ports.
GatewayPorts can be used to specify
that sshd should allow remote port forwardings to bind to non-loopback
addresses, thus allowing other hosts to connect. The argument may be
no to force remote port forwardings to
be available to the local host only,
yes to force remote port forwardings to
bind to the wildcard address, or
clientspecified to allow the client to
select the address to which the forwarding is bound. The default is
no.

Determines whether to be strict about the identity of the GSSAPI acceptor
a client authenticates against. If set to
yes then the client must authenticate
against the host service on the current hostname. If set to
no then the client may authenticate
against any service key stored in the machine's default store. This
facility is provided to assist with operation on multi homed machines. The
default is yes.

Specifies the key types that will be accepted for hostbased authentication
as a list of comma-separated patterns. Alternately if the specified value
begins with a ‘+’ character, then the specified key types
will be appended to the default set instead of replacing them. If the
specified value begins with a ‘-’ character, then the
specified key types (including wildcards) will be removed from the default
set instead of replacing them. The default for this option is:

Specifies whether or not the server will attempt to perform a reverse name
lookup when matching the name in the
~/.shosts,
~/.rhosts, and
/etc/hosts.equiv files during
HostbasedAuthentication. A setting of
yes means that
sshd(8) uses the name supplied
by the client rather than attempting to resolve the name from the TCP
connection itself. The default is
no.

Specifies a file containing a public host certificate. The certificate's
public key must match a private host key already specified by
HostKey. The default behaviour of
sshd(8) is not to load any
certificates.

Identifies the UNIX-domain socket used to communicate with an agent that
has access to the private host keys. If the string
“SSH_AUTH_SOCK” is specified, the location of the socket
will be read from the SSH_AUTH_SOCK
environment variable.

Specifies the IPv4 type-of-service or DSCP class for the connection.
Accepted values are af11,
af12,
af13,
af21,
af22,
af23,
af31,
af32,
af33,
af41,
af42,
af43,
cs0,
cs1,
cs2,
cs3,
cs4,
cs5,
cs6,
cs7,
ef,
lowdelay,
throughput,
reliability, a numeric value, or
none to use the operating system
default. This option may take one or two arguments, separated by
whitespace. If one argument is specified, it is used as the packet class
unconditionally. If two values are specified, the first is automatically
selected for interactive sessions and the second for non-interactive
sessions. The default is af21
(Low-Latency Data) for interactive sessions and
cs1 (Lower Effort) for non-interactive
sessions.

Specifies whether to allow keyboard-interactive authentication. The
argument to this keyword must be yes or
no. The default is to use whatever
value ChallengeResponseAuthentication
is set to (by default yes).

Specifies whether the password provided by the user for
PasswordAuthentication will be
validated through the Kerberos KDC. To use this option, the server needs a
Kerberos servtab which allows the verification of the KDC's identity. The
default is no.

Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms
must be comma-separated. Alternately if the specified value begins with a
‘+’ character, then the specified methods will be appended
to the default set instead of replacing them. If the specified value
begins with a ‘-’ character, then the specified methods
(including wildcards) will be removed from the default set instead of
replacing them. The supported algorithms are:

The optional rdomain qualifier requests
sshd(8) listen in an explicit
routing domain. If port is not specified,
sshd will listen on the address and all
Port options specified. The default is
to listen on all local addresses on the current default routing domain.
Multiple ListenAddress options are
permitted. For more information on routing domains, see
rdomain(4).

Gives the verbosity level that is used when logging messages from
sshd(8). The possible values
are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and
DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and
DEBUG3 each specify higher levels of debugging output. Logging with a
DEBUG level violates the privacy of users and is not recommended.

Specifies the available MAC (message authentication code) algorithms. The
MAC algorithm is used for data integrity protection. Multiple algorithms
must be comma-separated. If the specified value begins with a
‘+’ character, then the specified algorithms will be
appended to the default set instead of replacing them. If the specified
value begins with a ‘-’ character, then the specified
algorithms (including wildcards) will be removed from the default set
instead of replacing them.

The algorithms that contain “-etm” calculate the MAC after
encryption (encrypt-then-mac). These are considered safer and their use
recommended. The supported MACs are:

Introduces a conditional block. If all of the criteria on the
Match line are satisfied, the keywords
on the following lines override those set in the global section of the
config file, until either another Match
line or the end of the file. If a keyword appears in multiple
Match blocks that are satisfied, only
the first instance of the keyword is applied.

The arguments to Match are one or more
criteria-pattern pairs or the single token
All which matches all criteria. The
available criteria are User,
Group,
Host,
LocalAddress,
LocalPort,
RDomain, and
Address (with
RDomain representing the
rdomain(4) on which the
connection was received.)

The match patterns may consist of single entries or comma-separated lists
and may use the wildcard and negation operators described in the
PATTERNS section of
ssh_config(5).

The patterns in an Address criteria may
additionally contain addresses to match in CIDR address/masklen format,
such as 192.0.2.0/24 or 2001:db8::/32. Note that the mask length provided
must be consistent with the address - it is an error to specify a mask
length that is too long for the address or one with bits set in this host
portion of the address. For example, 192.0.2.0/33 and 192.0.2.0/8,
respectively.

Specifies the maximum number of open shell, login or subsystem (e.g. sftp)
sessions permitted per network connection. Multiple sessions may be
established by clients that support connection multiplexing. Setting
MaxSessions to 1 will effectively
disable session multiplexing, whereas setting it to 0 will prevent all
shell, login and subsystem sessions while still permitting forwarding. The
default is 10.

Specifies the maximum number of concurrent unauthenticated connections to
the SSH daemon. Additional connections will be dropped until
authentication succeeds or the
LoginGraceTime expires for a
connection. The default is 10:30:100.

Alternatively, random early drop can be enabled by specifying the three
colon separated values start:rate:full (e.g. "10:30:60").
sshd(8) will refuse connection
attempts with a probability of rate/100 (30%) if there are currently start
(10) unauthenticated connections. The probability increases linearly and
all connection attempts are refused if the number of unauthenticated
connections reaches full (60).

Multiple permissions may be specified by separating them with whitespace. An
argument of any can be used to remove
all restrictions and permit any listen requests. An argument of
none can be used to prohibit all listen
requests. The host name may contain wildcards as described in the PATTERNS
section in
ssh_config(5). The
wildcard ‘*’ can also be used in place of a port number to
allow all ports. By default all port forwarding listen requests are
permitted. Note that the GatewayPorts
option may further restrict which addresses may be listened on. Note also
that ssh(1) will request a
listen host of “localhost” if no listen host was
specifically requested, and this this name is treated differently to
explicit localhost addresses of “127.0.0.1” and
“::1”.

Multiple forwards may be specified by separating them with whitespace. An
argument of any can be used to remove
all restrictions and permit any forwarding requests. An argument of
none can be used to prohibit all
forwarding requests. The wildcard ‘*’ can be used for host
or port to allow all hosts or ports, respectively. By default all port
forwarding requests are permitted.

Specifies whether root can log in using
ssh(1). The argument must be
yes,
prohibit-password,
forced-commands-only, or
no. The default is
prohibit-password.

If this option is set to
prohibit-password (or its deprecated
alias, without-password), password and
keyboard-interactive authentication are disabled for root.

If this option is set to
forced-commands-only, root login with
public key authentication will be allowed, but only if the
command option has been specified (which
may be useful for taking remote backups even if root login is normally not
allowed). All other authentication methods are disabled for root.

Specifies whether ~/.ssh/environment
and environment= options in
~/.ssh/authorized_keys are processed by
sshd(8). Valid options are
yes,
no or a pattern-list specifying which
environment variable names to accept (for example
“LANG,LC_*”). The default is
no. Enabling environment processing may
enable users to bypass access restrictions in some configurations using
mechanisms such as LD_PRELOAD.

Specifies the key types that will be accepted for public key
authentication as a list of comma-separated patterns. Alternately if the
specified value begins with a ‘+’ character, then the
specified key types will be appended to the default set instead of
replacing them. If the specified value begins with a ‘-’
character, then the specified key types (including wildcards) will be
removed from the default set instead of replacing them. The default for
this option is:

Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated, optionally followed a maximum amount of time
that may pass before the session key is renegotiated. The first argument
is specified in bytes and may have a suffix of ‘K’,
‘M’, or ‘G’ to indicate Kilobytes, Megabytes,
or Gigabytes, respectively. The default is between ‘1G’ and
‘4G’, depending on the cipher. The optional second value is
specified in seconds and may use any of the units documented in the
TIME FORMATS section.
The default value for RekeyLimit is
default none, which means that rekeying
is performed after the cipher's default amount of data has been sent or
received and no time based rekeying is done.

Specifies revoked public keys file, or
none to not use one. Keys listed in
this file will be refused for public key authentication. Note that if this
file is not readable, then public key authentication will be refused for
all users. Keys may be specified as a text file, listing one public key
per line, or as an OpenSSH Key Revocation List (KRL) as generated by
ssh-keygen(1). For more
information on KRLs, see the KEY REVOCATION LISTS section in
ssh-keygen(1).

Specifies an explicit routing domain that is applied after authentication
has completed. The user session, as well and any forwarded or listening IP
sockets, will be bound to this
rdomain(4). If the routing
domain is set to %D, then the domain in
which the incoming connection was received will be applied.

Specifies one or more environment variables to set in child sessions
started by sshd(8) as
“NAME=VALUE”. The environment value may be quoted (e.g. if
it contains whitespace characters). Environment variables set by
SetEnv override the default environment
and any variables specified by the user via
AcceptEnv or
PermitUserEnvironment.

Sets the octal file creation mode mask (umask) used when creating a
Unix-domain socket file for local or remote port forwarding. This option
is only used for port forwarding to a Unix-domain socket file.

The default value is 0177, which creates a Unix-domain socket file that is
readable and writable only by the owner. Note that not all operating
systems honor the file mode on Unix-domain socket files.

Specifies whether to remove an existing Unix-domain socket file for local
or remote port forwarding before creating a new one. If the socket file
already exists and
StreamLocalBindUnlink is not enabled,
sshd will be unable to forward the port
to the Unix-domain socket file. This option is only used for port
forwarding to a Unix-domain socket file.

Specifies whether sshd(8)
should check file modes and ownership of the user's files and home
directory before accepting login. This is normally desirable because
novices sometimes accidentally leave their directory or files
world-writable. The default is yes.
Note that this does not apply to
ChrootDirectory, whose permissions and
ownership are checked unconditionally.

Specifies whether the system should send TCP keepalive messages to the
other side. If they are sent, death of the connection or crash of one of
the machines will be properly noticed. However, this means that
connections will die if the route is down temporarily, and some people
find it annoying. On the other hand, if TCP keepalives are not sent,
sessions may hang indefinitely on the server, leaving
“ghost” users and consuming server resources.

The default is yes (to send TCP keepalive
messages), and the server will notice if the network goes down or the
client host crashes. This avoids infinitely hanging sessions.

Specifies a file containing public keys of certificate authorities that
are trusted to sign user certificates for authentication, or
none to not use one. Keys are listed
one per line; empty lines and comments starting with
‘#’ are allowed. If a certificate is
presented for authentication and has its signing CA key listed in this
file, then it may be used for authentication for any user listed in the
certificate's principals list. Note that certificates that lack a list of
principals will not be permitted for authentication using
TrustedUserCAKeys. For more details on
certificates, see the CERTIFICATES section in
ssh-keygen(1).

Specifies whether X11 forwarding is permitted. The argument must be
yes or
no. The default is
no.

When X11 forwarding is enabled, there may be additional exposure to the
server and to client displays if the
sshd(8) proxy display is
configured to listen on the wildcard address (see
X11UseLocalhost), though this is not
the default. Additionally, the authentication spoofing and authentication
data verification and substitution occur on the client side. The security
risk of using X11 forwarding is that the client's X11 display server may
be exposed to attack when the SSH client requests forwarding (see the
warnings for ForwardX11 in
ssh_config(5)). A system
administrator may have a stance in which they want to protect clients that
may expose themselves to attack by unwittingly requesting X11 forwarding,
which can warrant a no setting.

Note that disabling X11 forwarding does not prevent users from forwarding
X11 traffic, as users can always install their own forwarders.

Specifies whether sshd(8)
should bind the X11 forwarding server to the loopback address or to the
wildcard address. By default, sshd binds the forwarding server to the
loopback address and sets the hostname part of the
DISPLAY environment variable to
localhost. This prevents remote hosts
from connecting to the proxy display. However, some older X11 clients may
not function with this configuration.
X11UseLocalhost may be set to
no to specify that the forwarding
server should be bound to the wildcard address. The argument must be
yes or
no. The default is
yes.

OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen.
Aaron Campbell,
Bob Beck,
Markus Friedl,
Niels Provos,
Theo de Raadt and
Dug Song removed many bugs, re-added newer
features and created OpenSSH. Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.
Niels Provos and
Markus Friedl contributed support for
privilege separation.